It looks from the photos that the pilot's head is quite high up, so maybe the sim is pretty accurate already.
Each mirror requires a complete 3D scene render, just like the view out the front window, the end result is just smaller. It still has to go through all of the same logic. Do you mean a rear view camera mode for the MFD? That wouldn't have the same fps impact as an additional mirror, EECH is already rendering views for LLLTV and FLIR. It could be added as a selection in the MFD modes.